In this context, an erasure means a faulty set having a known position in the stream, and is thus different from an error which indicates a faulty set having an unknown position in the stream. Such a method is already known in the art, e.g. from the book `Theory and practice of error control codes`, by R. E. Blahut, published by Addison-Wesley Publishing Company, Reading, 1983, pp. 11 and 199. Therein, it is used in a receiver which processes the stream of sets of digital signal values and declares a set erased either when it is received ambiguously (p. 11), or when presence of interference or a transient malfunction is detected (p. 11), or when various internal validity checks fail (p. 199). However, from this book it is not clear at all which criterion should be used to decide when a set is received ambiguously, or when interference or a transient malfunction is present, or which internal validity checks should be performed.
The advantage of being able to detect whether a set is erased or not becomes apparent when the stream of digital signal values is encoded according to an error-correcting code having a so-called minimum distance d. Indeed, in that case, a number of R errors and E erasures in this bit stream may be corrected when 2.times.R+E+1.ltoreq.d. Thus, by detecting erasures the error correcting capability of the code is doubled for a given minimum distance d. This may be appreciated from the fact that half the error correcting work, specifically the work locating faulty digital signal values in the stream, is already performed when an erasure is detected.